Changing wallpapers in GNOME 3.4 could be a bit confusing initially
because there are three different wallpaper settings and there is no
simple, convenient and uniform way to change these various wallpapers.
There is a wallpaper setting for your desktop, another one for the login
screen, and yet another one for the lock screen. This article explains
how to set each wallpaper on a Debian 7.0 (Wheezy) system.

Resources

The next three sections explain the steps required to change each
wallpaper. A different image is used for each wallpaper. In the commands
and screenshots that appear in this article, the following three images
are used.

You should be able to see the updated desktop wallpaper as soon as you
execute the command. No service needs to be restarted.

Updated desktop wallpaper

Change login wallpaper

To change the login wallpaper, first become a superuser by running the
su command and entering the root password. Then invoke a
new shell as the user that displays the GNOME 3 login screen. This is
'Debian-gdm' on Debian GNU/Linux 7.0 (Wheezy).

su - Debian-gdm -s /bin/bash

On other Linux systems, this user ma be 'gdm' instead, so you may have
to replace 'Debian-gdm' with 'gdm' in the above command on other Linux
systems. To determine which user is displaying the GNOME 3 login screen,
run the following command before logging in to your desktop while the
login screen is displayed: ps -ef | grep gnome-session.
The first column of the output displays the user running the login
screen. If it's a numeric ID, look for that ID in /etc/passwd to
determine the username.

You might want to edit the attributes of the size element
in the XML to match the image size. You may see other XML files
in the same directory to see what these XML files look like. If we
assume that your XML file with the path to the image for your lock
screen wallpaper is saved as
/usr/share/images/desktop-base/lockscreen.xml, then you have to run the
following command so that /etc/alternatives/desktop-background.xml is
updated as a symbolic link to your new XML file.